A push button switch is conventionally used for operating an electric circuit in a variety of electric and electronic appliances such as electronic calculators, telephones, personal computers, wordprocessors and remote control devices for various appliances. In the prior art, keypads having a plurality of push button switches have been simultaneously and integrally formed from a rubber material such as silicone rubber or the like. Each of these push button switches includes a key top formed from a plastic material which is fitted on the surface of the keypad. Recently, the plastic key top has been replaced with a key top formed from a rubber material in order to improve the appearance, design and feel of the push button switch.
Such rubber key tops are generally made in such a manner that a plurality of the rubber key tops, to which several colors such as red, blue, yellow and the like are applied, are arranged in a single rubber cover. For example, when the push button switch is used for an electronic calculator, a remote control device or the like, twenty to thirty rubber push button keys are integrally formed so as to cover a printed circuit board having a pectinate electrode pattern. For this reason, the plurality of rubber push button switches are referred to as a rubber cover switch in the lump. Alternatively, the rubber key top may be constructed so as to act as a display key top on which a desired pattern is provided, such as a letter or the like.
The above-described conventional colored key tops are made by arranging a rubber compound, blended with a pigment of a predetermined color, on a predetermined surface portion of the key top in a tessellate pattern and subjecting it to a pressing operation. Unfortunately, the blending of the rubber compound with the pigment causes the coloration of the key tops in each production lot to be highly troublesome. In addition, the conventional manufacturing process causes the rubber material to flow out of the predetermined surface section of the key top during the pressing operation, resulting in defects such as the dislocation of the pattern or the like. This not only leads to the push button switches having an inferior appearance and decreased productivity, but to a decreased degree of design freedom as well.
A pattern such as a letter or the like provided on the key top is generally formed by applying a rubber ink blended with a coloring pigment onto the surface of the key top by wet printing, such as screen printing or intaglio printing, to form a pattern which projects from the key top by a predetermined thickness. Screen printing leads to the formation of a pattern having a thickness of 10-20 um, whereas the intaglio offset printing results in a pattern having a thickness of 1-10 um. Whether screen printing or intaglio offset printing is selected depends upon the strength of adherence of the rubber ink on the key top and the resistance to wear of the rubber ink. Unfortunately, the pattern formed using the rubber ink does not exhibit good wear resistance. Therefore, when the key top is tapped or touched repeatedly, the projecting pattern is worn or damaged, thereby reducing the durability of the push button switch.
Accordingly, it would be highly desirable to develop a push button switch which is capable of being manufactured with improved productivity and which enables an increased degree of freedom in selecting the design formed thereon.